Usage is a
guide on how to use something properly; use means “one
or more instances of using something,” or “function.” Thus the
use of a semicolon is to
separate clauses, while its usage is the list of rules on exactly how it has to be used.
Someone who knows the use of a word understands how it
fits in a sentence; someone who knows the usage has
studied the grammatical rules, semantic relations, and
appropriate mechanics. Each time
you use something, that's one use (the noun), not one
usage. Unless you're talking about grammar, you usually
mean use rather than usage: don't use the
longer word just because it sounds more impressive.
[Revised 14 July 2000]